This book offers the first comprehensive edition and translation of all surviving versions—Latin, Armenian, Arabic, and Aljamiado—of the polemical correspondence attributed to the Byzantine emperor Leo III and the Umayyad caliph 'Umar II. Far from simple diplomatic communication, these letters form part of a centuries-long Christian–Muslim exchange, rooted in fictional authorship but widely circulated across the Mediterranean from the eighth to the sixteenth century. The book explores their multilingual transmission and textual fluidity, as well as the evolution of their arguments, especially regarding scriptural reliability and Christology, to demonstrate how diverse communities adapted the texts to local polemical contexts. It identifies three main textual groupings and traces recurring argumentative strands, many of which derive from specific Qur'anic passages, suggesting their origins in an oral, cross-confessional polemical milieu. The correspondence not only reflects shared themes of religious disputation but also continuously imagines itself as one episode in a larger, unending dialogue between Christianity and Islam. By situating these texts within vibrant Mediterranean networks, the book provides crucial insights into the construction, adaptation and transmission of polemical literature in the premodern world.